This invention relates generally to the processing of lithographic plates and, more particularly, to method and apparatus for automatically inspecting a lithographic plate after it has been exposed and bent to determine whether an image on the plate is in proper registration with respect to the plate.
In printing a lithographic color image on newspaper stock, for example, stock is fed through successive rolls of a press. One or more lithographic plates are mounted on each roll for printing a single color of the image on the stock, with different color plates on successive rolls adding different colors to the image. To produce a clear final image, it is essential that precise registration of the images be achieved; otherwise the superimposed printed color images on the stock will be out of registration. Obtaining such registration is difficult. Not only must the image be precisely applied to the plate during the exposure process, but the exposed plate must be precisely bent to form the flanges required for mounting the plate on the press. Over the years plates have been checked for proper image registration in various ways, but these have proven to be less than entirely satisfactory, either because the inspection has occurred prior to the time the plate is in final form (e.g., prior to the bending step), thus making a final check of registration impossible, or because the inspection has involved manual steps, such as manually conveying a plate after it has been bent to an inspection station where it is inspected, and then manually removing the plate from the inspection station, all of which greatly increases the time and labor required before the plate is ready for application to a press.